1. Field of the Invention
This invention relates to new and improved methods of fluidic support. In particular, this invention relates to new and improved methods of (1) counteracting the gravitational bending moment present in a substrate tube during a modified chemical vapor deposition preform fabrication process, and (2) improved cooling of the substrate tube to increase the thermophoretic force gradient. Hence, use of the methods of this invention results in a straight optical fiber preform, with greater deposition efficiency.
2. Description of the Prior Art
The modified chemical vapor deposition (MCVD) process used to produce optical fibers utilizes a quartz substrate tube that is mounted in a glass working lathe. The substrate tube is chucked at one end and is supported by graphite vee blocks or nylon or Teflon rollers at the opposite end. The substrate tube is then heated at the chucked end and straightened to the center line of the lathe. A larger tube is inserted in another chuck and sealed to the substrate tube. The supports are now removed.
A torch assembly is mounted on a fire carriage that traverses the length of the lathe bed that heats the exterior of the substrate tube. Each traversal commences at the portion of the tube closest to the chemical input end, and is termed a pass. The heat that is produced by the torch assembly is required to cause the chemicals on the interior of the tube to react, and also to fuse or sinter the glass particles that are deposited on the inner wall of the tube. Temperatures up to 2100.degree. C. are attained during the course of the process, well above the softening point (1813.degree. C.) of fused quartz. The MCVD process typically requires over 60 torch passes and eight to twelve hours to complete. The tube is rotated during the process to maintain circular symmetry, but this rotation does not provide any force to center the tube. On the contrary, in fact, rotation causes any existing eccentricity to increase because of centrifugal force.
Disadvantageously, the sagging of a rotating quartz tube occurs at the extreme ends thereof, especially the end at which the heating is initiated. Such sagging is the result of the gravitational bending moment and small deviations normally present in the tube.
The heat zone produced by the torch assembly is located between the two supporting chucks for the quartz tube. When the substrate tube is heated above the softening point at either end, the effect of the nearest supporting chuck is negated, while the bending moment is the greatest. Since so many passes are performed, the problem increases with each pass, and results in a wavy or eccentric preform. The eccentricity also causes asymmetric heating, compounding the problems of temperature control and noncircularity of the deposited material. The waviness of the preform reduces the yield of the preform in either the deposition process or the draw process. Noncircular deposition leads to cores that are not concentric with the outside of the cladding, and may have unacceptable, out-of-tolerance dimensions.
U.S. Pat. No. 4,263,032, issued Apr. 21, 1981 to Sinclair et al., and U.S. Pat. No. 4,302,230, issued Nov. 24, 1981 to MacChesney et al., relate to making optical fiber preforms more expeditiously by enhancing the thermophoretic deposition force. The '032 patent suggests a fluid stream as a cooling means. The '230 patent suggests water for cooling. Disadvantageously, the '230 patent prefers de-ionized water in order to avoid introduction of contaminants which may contribute to a lowering of the strength of the resultant fiber.
Direct physical support of a glass surface by a material in contact with it causes problems by introducing defects and contamination into the surface of the preform. Unfortunately, there appears to be no material known that can resist the temperatures involved and not damage the surface of the preform by scratching or contamination. As is well recognized, brittle materials such as silica glass are strongly dependant on the integrity of the surface for strength, with an essentially perfect surface required to retain the intrinsic high strength. It is well known that physical contact of a glass surface by a solid object will leave defects in the surface that are difficult to heal, and sometimes particles will be left behind, generating a low strength site for future failure. A defect can readily reduce the strength of glass fiber after drawing by two orders of magnitude. In addition, any particle present on the preform surface will have its size magnified, relative to the fiber, during the drawing process, thus creating a larger defect, and a probable low strength failure point.
Liquids can be considered for a coolant to aid in the thermophoretic force as indicated in the patent of MacChesney, but the addition of a cooling water stream to the apparatus is difficult. In addition, the water must be of very high quality, such as freshly deionized or distilled, to avoid leaving any residue that will act to harm the surface.